A typical cathode ray picture tube includes a source of electron beams which scan a face plate or screen. As is known, if the scan lines traverse the screen at constant speed, the display on the picture tube will not be linear since as the electron beams scans the screen, it is working across an arc of a circle. Hence the scan will be traveling for a shorter distance across the center of the screen as compared to either edge of the tube or screen. Accordingly, if the beam is traveling at one rate of speed across the screen, there will be a non-linear display.
While the linearity of display is of some interest with respect to regular television picture tubes, it is difficult to see any or to discern any differences if the scan is not exactly as desired. However, the linearity of the display is more critical for computer terminals such as where typewritten pages are displayed on the face of the picture tube and if the type on one edge of the screen is smaller than the type on the other edge of the screen, it will be readily apparent and undesirable.
In addition, component tolerance (deflection yoke, horizontal output transformer, transistors, tubes, etc) will cause additional variations on the sides of the screen.